From: Christoph Hellwig <hch@lst.de>
To: Michael Tokarev <mjt@tls.msk.ru>
Cc: Kevin Wolf <kwolf@redhat.com>,
Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>,
Qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>,
Christoph Hellwig <hch@lst.de>
Subject: Re: [Qemu-devel] Default cache mode
Date: Wed, 29 Jun 2011 16:50:08 +0200 [thread overview]
Message-ID: <20110629145008.GB17085@lst.de> (raw)
In-Reply-To: <4E0B33DE.5000706@msgid.tls.msk.ru>
On Wed, Jun 29, 2011 at 06:17:02PM +0400, Michael Tokarev wrote:
> > Honestly, I don't know. Usually the problem is resolved with setting a
> > different cache option, so nobody bothers to ask for details. I'd guess
> > that it's ext4 in most cases.
>
> Extremly poor performance also happens on raw devices --
> be it lvm volumes or plain partitions, so that's w/o
> any filesystem.
Depends on your setup. If you have ATA devices that have WCE=1
it will suck due to the non-queueable FLUSH command. If you have
an older kernel with the draining barriers semantics it will to.
If you run on a SAS disk or FC array with WCE=0 performance will
be quite good and close to native performance on these.
That's all assuming you use O_DIRECT. using cache=writhrough
as-is will suck everywher.
next prev parent reply other threads:[~2011-06-29 14:50 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-29 11:59 [Qemu-devel] Default cache mode Kevin Wolf
2011-06-29 12:06 ` Anthony Liguori
2011-06-29 12:16 ` Kevin Wolf
2011-06-29 12:23 ` Anthony Liguori
2011-06-29 12:32 ` Kevin Wolf
2011-06-29 13:51 ` Christoph Hellwig
2011-06-29 14:17 ` Michael Tokarev
2011-06-29 14:50 ` Christoph Hellwig [this message]
2011-06-29 15:07 ` Michael Tokarev
2011-06-29 13:50 ` Christoph Hellwig
2011-06-29 14:13 ` Anthony Liguori
2011-06-29 14:47 ` Christoph Hellwig
2011-06-29 16:03 ` Stefan Hajnoczi
2011-06-29 13:53 ` Frediano Ziglio
2011-06-29 14:11 ` Kevin Wolf
2011-06-29 14:12 ` Christoph Hellwig
2011-06-29 14:21 ` Kevin Wolf
2011-06-29 13:49 ` Christoph Hellwig
2011-06-29 12:08 ` Anthony Liguori
2011-06-29 13:55 ` Christoph Hellwig
2011-06-29 14:20 ` Anthony Liguori
2011-06-29 14:52 ` Christoph Hellwig
2011-06-29 15:26 ` Anthony Liguori
2011-06-29 12:21 ` Alexander Graf
2011-06-29 13:57 ` Stefan Hajnoczi
2011-06-29 14:05 ` Alexander Graf
2011-06-29 12:52 ` Stefan Hajnoczi
2011-06-29 13:56 ` Christoph Hellwig
2011-06-29 13:00 ` Avi Kivity
2011-06-29 13:05 ` Anthony Liguori
2011-06-29 13:30 ` Avi Kivity
2011-06-29 13:11 ` Kevin Wolf
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